Implantable nerve stimulator and method of use



United States Patent [72] Inventors: Seymour I. Schwartz Rochester, New York;

Robert C. Wingrove, Minneapolis, Minnesota [21] Application No.: 800,044

[22] Filed: Feb. 13, 1969 [45] Patented: Aug. 4, 1970 Continuation of Ser. No, 397,899, Sept. 21, 1964, now

' abandoned.

[73] Assignee: Medtronic, Inc., Minneapolis,

Minnesota (21 part interest) a corporation of Minnesota.

[54] IMPLANTABLE NERVE STIMULATOR AND METHOD OF USE 18 Claims, 13 Drawing Figs.

[52] 11.8. C1. 128/419 [51] Int. Cl. A61n H18 [50] Field ofSearch 128/2.1,

[56] References Cited UNITED STATES PATENTS 2,532,788 12/1950 Sarnoff 3,157,181 11/1964 McCarty 3,236,240 2/1966 Bradley 3,253,596 3/1966 Keller,.1r.

Primary Examiner- William E. Kamm Attorney-Lew Schwartz & Donald R. Stone ABSTRACT: An apparatus for stimulating the carotid sinus nerve, being entirely implantable in an animal, having a battery powered timed pulse producing circuit, a pair of electrodes adapted to be atraumatically connected to spaced points on the exterior surface of the nerve to be stimulated, and capacitive coupling between the electrodes and the pulse producing circuit to provide a substantially zero net DC current flow through the nerve.

Patented Aug. 4, 1970 3,522,811

Sheet 1 of 3 INVENTORS Sz na we Z Sara/4x72 BY P055427 6'. k/IAI6ROV6 US. PATENT 3,522,811 IMPLANTABLE NERVE STIMULATGR AND METHOD OF USE This application is a continuation of application Ser. No. 397,899, filed Sept. 21; 1964 and now abandoned.

This invention relates to a method and means for electrically stimulating the carotid sinus nerve to control blood pres sure in hypertensive persons, and more particularly to such means comprising an implantable stimulator atraumatically connected directly to the carotid sinus nerve to supplement carotid sinus nerve impulses with artificial electrical impulses which will be carried to the vasomotor center to control systemic blood pressure. As used herein, the word atraumatic has its normal meaning of not inflicting or causing damage or injury.

In some ninety percent of hypertensive patients, the cause of hypertension is unknown. However, it is known that in the human body there are baroreceptor areas which reflexly contribute to the control of systemic blood pressure. At least four of these major baroreceptor areas have been identified. In ad dition to the two that are located at the origin of the internal carotid arteries bilaterally (carotid sinuses), there are two situated in the aortic arch or in the major branches arising from this arch. It is known that in the hypertensive patient the baroreceptor mechanism located at the branching of the internal and external carotid sinus arteries do not function as they do in the normotensive person.

The carotid sinus baroreceptor endings are best considered as stretch receptors". These baroreceptors are nerve endings in the wall of the carotid artery just below the car on each side of the head. Their function is to cause impulses to be transmitted along the carotid sinus nerve whose frequency is in direct proportion to the stretching of the arterial walls, which in turn is proportional to the blood pressure within the artery. Physical deformation of the cellular wall probably affects an increasing negative depolarization of the cell membrane until the impulse threshold is reached and an action potential is produced. The frequency of these impulses is also somewhat dependent upon the rate of change of pressure within the artery. The carotid sinus nerve serves as the afferent limb of the baroreceptor reflex and transmits centrally to the vasomotor center those action potentials which arise from the carotid baroreceptors. These afferent impulses act centrally to inhibit sympathetic discharge by the vasomotor center and thereby secondarily affect vascular adjustments. The sympathetic nerves, which constitute the efferent limb, complete the carotid sinus reflex circuit and carry impulses which alter peripheral vascular resistance, cardiac output, and venous return.

An increase in the intraluminal pressure of the carotid baroreceptor reflexly, through the vasomotor center, lowers blood pressure, induces bradycardia, decreases cardiac output, and diminishes venous return. When the pressure within the baroreceptor is lowered, the converse situations are noted. The baroreceptor impulses, whose frequency is directly proportional to the intra-arterial pressure and to the rate of rise of this pressure within the baroreceptor organ, travel via the carotid sinus nerves to the hypothalamus area of the brain where they act upon the sympathetic nerve system which controls the diameters of various arteries and arterioles through the body.

In the normotensive person, the frequency ofthese impulses varies approximately from to 100 impulses per second as the pressure varies from 0 to 200mm ofmer'cury. In the hypertensive person, the baroreceptive centers-continue to be functional, but the threshold stimulus required to trigger the baroreceptor is significantly elevated. In chronic hypertension; there are'actually fewer impulses along the carotid sinus nerve than in the normotensive situation. .Thus, in the hypertensive person the baroreceptors'are essentially desensitized, in that the impulse frequency is lower for a given pressure than in the normotensive person.

In treating hypertensive patients, therefore, it is desirable to correct or artificially compensate for the desensitized baroreceptors. Surgical procedures directed at the reversal of hypertension have varied in their approach, but all have been concerned with counteracting an increased peripheral vascular resistance. More recently, attention has been directed away from the neurogenic factor and toward humoral factors eminating from the kidney. Renal artery reconstruction, in appropriate cases, and nephrectomy in others have had dramatic results, but these procedures are not applicable to all hypertensive patients. This invention of implantable artificial electrical means to selectively stimulate the carotid sinus nerve of the baroreceptor system provides a new approach to reducing systemic arterial hypertension. While the invention has been described as being applied to human beings, it could, of course, beadapted to reduce hypertension in other living animals. In fact, the preliminary experimental testing was conducted on dogs.

Electrical impulses for major organs of the body may be transmitted to that organ by means of pointed electrodes which penetrate the body of the organ and indirectly make contact with a nerve network without damaging the organ. Such general stimulation is not effective where the impulse must be superimposed on one particular nerve fiber and that fiber alone. The present structure is designed to transmit an electrical impulse specifically and solely to a surgically exposed carotid sinus nerve. Once the carotid sinus nerve has been exposed, the bipolar electrodes are placed around the nerve fiber and sewn closed to avoid penetration and damage to the nerve fiber itself. Because a net DC current would be destructive to the nerve and the electrodes, the present structure is designed to provide a substantially zero net DC current flow.

Artificial implantable stimulators in present use have been restricted in their use by reason of their size and bulk. The location of the carotid sinus nerve in the upper neck and head itself prevents implantation of presently used stimulators in the immediate area. The present invention which can operate on power from small rechargeable batteries within the stimulator unit occupies less than one-half the space of the cardiac pacemaker disclosed by Greatbatch. The present invention is of proper size to be seated within the limited confines of the mastoid cavity or locally just under the skin. Further, the present invention includes apparatus which prevents significant net DC current flow through the nerve.

It is a principal object of this invention to provide a practical method and means to artificially electrically stimulate the carotid sinus nerves to provide long-term supression of hypertension.

It is another object of this invention to provide an implantable stimulator which will provide long-term supresson of hypertension by artificially adding electrical impulses to the carotid sinus nerves with a net DC current flow through the nerves of substantially zero.

It is a further object of the present invention to provide a battery-powered implantable stimulator which will provide long'term suppression of hypertension by artificially adding electrical impulses to the carotid sinus nerves.

A still further object of this invention is to provide an implantable stimulator employing a recharge system to recharge the power supply located within the implantable stimulator.

It is a still further object of the invention to provide an implantable stimulator employing a portable recharge system to recharge the power supply located within the implantable stimulator.

In the drawings,

FIG. 1 is an elevational view of an implantable stimulator made according to the present invention;

FIG. 2 is a diagrammatic profile view of the head of a patient, partially in section, showing the relationship of a stimulator to the head when implanted in a mastoid cavity and connected to a carotid sinus nerve;

FIG. 3 is an enlarged fragmentary view of a portion of an implantable stimulator disclosing the stimulator electrodes in electrical contact with a carotid sinus nerve;

FIG. 4 is a sectional view taken on line 4 4 in FIG. 3;

FIG. is a view of an alternative shape of stimulator unit made according to the present invention;

FIG. 6 is a diagrammatic profile view of the head of a patient, partially in section, showing the relationship of the alternative shape stimulator unit of FIG. 5 implanted under the skin;

FIG. 7 is a view showing the recharging unit coil of FIG. 8 strapped in a position on a patients head for recharging;

FIG. 8 is a perspective view of a recharging unit including the recharging unit coil of FIG. 7;

FIG. 9 is a block diagram of an implantable stimulator pulse circuit deriving its power from rechargeable batteries.

FIG. I0 is a block diagram of an implantable stimulator pulse circuit deriving its power from replaceable batteries within the stimulator.

FIG. ll is a schematic circuit diagram showing a specific electrical pulse generating circuit of the present invention for use within a stimulator unit;

FIG. I2 is a block diagram of a stimulator recharging unit deriving its power from an A. C. line source; and

FIG. 13 is a block diagram of a portable stimulator recharging unit deriving its power from batteries.

Referring to the drawings and the numerals of reference thereon, in a first form of the invention as seen in FIGS. l, 2, 3

and 4 the implantable stimulator indicated generally at 1 which has been implanted under skin 2 in the mastoid cavity 3 of a hypertensive patient 4 comprises a pulse generating package 5 connected to electrodes 6 and 7 by flexible electrical conductors 8 and 9. The pulse generating package 5 is encased in a protective mass 13 composed of a synthetic resinous material such as silicon rubber which is compatible with the human body. The conductors are similarly encased in a protective sheath 10, and the electrodes 6 and 7 are partially similarly encased in protective covering 11. Electrodes 6 and 7 are partial cylindrical sections of an electrically conductive material compatible to the human body by being only partial cylinders, electrodes 6 and 7 define longitudinal apertures or slots large enough to receive nerve 12. Electrodes 6 and 7 are placed around exposed carotid sinus nerve 12 and pressed into electrical contact therewith. A flap 14 which is part of protective covering 11 is layed over the exposed portion of electrodes 6 and 7. Flap 14 makes contact with tab 15 which is also part of protective covering 11. Flap 14 is then sewn to tab 15 with surgical stitching 16 to firmly secure electrodes 6 and 7 to carotid sinus nerve l2 and to insulate electrodes 6 and 7 from other body tissues.

Referring to FIGS. 5 and 6 of the drawings, an alternative shape stimulator indicated generally at 21 is of thin construction as shown in FIG. 6 and is shown implanted under the skin 2 of a hypertensive patient 4 with electrodes 22 and 23 connected to the carotid sinus nerve 12.

Referring to FIG. 9 of the drawing, the implanted carotid sinus nerve stimulator is powered by rechargeable cells. lt can also be powered by a biological generator. Suitable rechargeable batteries 52 of this invention will operate continuously for approximately two weeks when fully charged. The battery power is supplied to an accurately timed pulse circuit 53 whose pulse output is directed through electrodes 6 and 7 to stimulate the carotid sinus nerve. The pulse output is capacitively coupled to electrodes 6 and 7 to block DC current flow from the nerve.

Referring to FIG. l0 ofthe drawings, a third type of stimulator unit which can be implanted inother areas such as the peetoral muscle or subcutaneously in the abdominal area, will be larger than those positioned in the area of the carotid sinus nerve and thus can be powered by replaceable primary batteries 48 such as mercury cells. This type of, stimulator has the disadvantage of requiring more extensive surgery and the'inconvenience of having long lead wires running subcutaneously up to the electrodes at the baroreceptor site. This type of unit of reasonable size using currently available mercury cells will operate approximately two years before battery replacement is necessary.

Referring to FIG. 11 of the drawings, a pulse circuit, indicated generally at 53, of the present invention is powered by battery 52. Battery 52 has one terminal connected to resistor 54, which is serially connected through capacitor 55 to base 56 of transistor 57. Battery 52 is also connected through a relatively large biasing resistor 58 to base 56 of transistor 57. Battery 52 is also connected directly to emitter 59 of transistor 57. The common junction of biasing resistor 58, capacitor 55, and base 56 of transistor 57 is connected through biasing resistor 60 to the second terminal of battery 52. Collector 61 of transistor 57 is connected through current limiting resistor 62 to the base 63 of transistor 64. Collector of transistor 64 is connected to the common junction of resistor 54 and capacitor 55. Storage capacitor 66 is serially connected between collector 65 and carotid sinus nerve electrode 6. Emitter 67 of transistor 64 is connected directly to carotid sinus nerve electrode 7 and to the battery side of biasing resistance 60.

With voltage of battery 52 impressed upon the circuit storage capacitor 66 will begin to charge through the circuit consisting of battery 52, resistor 54, electrodes 6 and 7, and the output load resistance comprising nerve 12 until collector 65 of transistor 64 reaches positive potential of battery 52 with respect to electrode 7. Between pulses collector 65 remains nearly at battery 52 potential with respect to electrode 7. The circuit 53 may be described as a free-running complementary multivibrator. A complete cycle of operation may be described as follows:

Biasing resistors 60 and 58, which form a voltage divider cause a small current to begin flowing through transistor 57, current limiting resistor 62, from base 63 to emitter 67 of transistor 64 and through battery 52. This small current is amplified by transistor 64 and current flows through resistor 54 causing the potential on collector 65 to lower with respect to the reference potential on electrode 7. The change in potential on collector 65 is coupled through capacitor 55 to base 56 of transistor 57 and causes an increase in condition of transistor 57 thereby leading to a further increase in conduction of transistor 64 and a further lowering of the potential on collector 65. Thus, capacitor 55 provides a positive feedback path which quickly results in saturation of transistor 64. After saturation of transistor 64, capacitor 55 becomes fully charged through the current path comprising emitter 59 and base 56 of transistor 57 and collector 65 and emitter 67 of transistor 64. At this point, with current no longer flowing through capacitor 55, transistor 57 begins to return to its original low conductive state provided by biasing resistors 60 and 58. However, as collector 65 of transistor 64 returns toward the positive battery potential because of the decrease in current through resistor 54, the charge on capacitor 55 completely cuts off transistor 57. Since no current is then flowing through resistor 62 to base 63 of transistor 64, transistor 64 is also completely cut off. Both transistors remain in the non-conducting state until capacitor 55 has discharged sufficiently through resistors 58 and 54 to allow biasing resistors 60 and 58 to bring transistor 57 into a conductive state and the cycle begins again. The stimulation output pulse occurs during the time transistor 64 is conducting, when storage capacitor 66 discharges through collector 65 and emitter 67 of transistor 64, through electrode I 7, the nerve, and back via electrode 6. Pulse width and pulse recurrence frequency are determined by the size of capacitor 55 and resistors 58 and 54, as well as by biasing resistor 60. The presence of capacitor 66, connected serially in the lead to electrode 6, makes the net DC current flow substantially zero for a complete cycle.

In actual use the circuit of FIG. 1 1 can be adjusted to produce a pulse amplitude of 0.5 to 5 volts, a pulse recurrence frequency of 60-100 pulses per second, and a pulse width from -200 microseconds with a battery voltage of 5 volts. While a specific circuit is shown, the pulse circuit can be any low current pulse circuit which produces no current during the off period.

Referring to FIGS. 7 and 8 of the drawings, a stimulator recharging unit indicated generally at 27 includes a radio frequency oscillator unit 28 electrically connected by insulated wire 29 to the primary recharging coil 30 encased in a suitable protective casing 31. When it is desired to recharge the battery 52 located within implantable stimulator unit 1 or 21 which has been implanted in the patient 4, the energized primary recharging coil 30 is placed in close proximity to the secondary recharging coil 42 located in implantable stimulator unit 1 or 21. Primary recharging coil 30 may be worn in a head band 32 or may be placed in proper position on or within a pillow, to accomplish recharging while the patient sleeps or rests.

Referring to FIGS. 9 and 1 l of the drawings, the battery 52 ofthe stimulator unit 1 may be periodically recharged by electromagnetic induction in secondary recharging coil 42 due to an electromagnetic field established by primary recharging coil 30 of FIG. 8. The radio frequency signal induced in the secondary recharging coil 42 is rectified by the rectifier circuit 43 including diode 44 and capacitor 45. The rectifier circuit output is coupled to battery 52 through resistor 56 and filtering capacitor 47.

Referring to FIGS. 12 and i3 of the drawings, the primary recharging coil 30 is energized by connection either to an alternating current power source 33 through a rectifier unit 24 and a radio frequency oscillator unit 28, or alternatively to batteries 34 and through a radio frequency oscillator 28.

We claim:

1. Apparatus for electrical stimulation of a selected nerve of an animal comprising: electrical circuit means including electrical output means; electrode means for atraumatic connection to the external surface of the nerve to be stimulated; electrically conductive means connecting said electrode means to said output means; said output means including means for producing a substantially zero net DC current flow through the nerve; said electrode means, said electrically conductive means and at least a portion of said electrical circuit means adapted to be implanted in the animal; and at least said portion of said electrical circuit means, at least a portion of said electrode means and said electrically conductive means covered by an electrically insulating substance substantially inert to body fluids and tissue.

2. The apparatus of Claim 1 in which: said electrode means comprises at least a pair of electrodes adapted to be connected to the external surface of the nerve in spaced relation.

3. The apparatus of Claim 2 in which: said electrodes each comprise a generally flat, electrically conductive material substantially inert to body fluids and tissue, and generally conformable to the external surface of the nerve.

4. The apparatus of Claim 1 in which: said means for producing a substantially zero net DC current flow comprises capacitance means connected intermediate at least a portion of said electrically conductive means and said electrical circuit means.

5. Apparatus for electrical stimulation of a selected nerve in an animal, the apparatus adapted to be-surgically implanted in the animal, comprising: first means for providing a timed electrical pulse; second means for producing a substantially zero net DC current flow; electrode means for atraumatic connection to the external surface of the nerve; said electrode means connected to said first means by said second means for producing a substantially zero net DC current flow through the nerve; and said first and second means and at least part of said electrode means being encapsulated in a substance substantially inert to body fluids and tissue. 1

6. The apparatus of Claim 5 in which said second means comprises a capacitor serially connected between said first means and at least a portion of said electrode means.

7. The apparatus of Claim 5 in which: said electrode means comprise at least a pair of electrodes adapted to be connected to the external surface of the nerve in spaced relation.

8. The apparatus of Claim 7 in which said electrodes each comprise: a generally flat, electrically conductive material substantially inert to body fluids and tissue; said material curving to form a partial cylinder defining an axial aperture extending the length of said cylinder; and said aperture being of sufficient width to permit atraumatic passage of a portion of the nerve therethrough.

9. The apparatus of Claim 5 in which said substance encapsulating at least a part of said electrodes includes: a further portion of said substance integral with said substance; said further portion adated to be folded over the exposed part of said electrodes when they are connected to the nerve, and fastened in the folded position.

10. Apparatus for electrical stimulation of a selected nerve of an animal and adapted to be implanted in the animal comprising: a timed electrical pulse producing circuit; a pair of spaced electrode means for atraumatic connection to the nerve, said electrode means adapted to be positioned in electrically conducting relation to the exterior surface of the nerve; electrical conducting means connecting said pulse producing circuit and one of said electrode means; capacitance means serially connected between said pulse producing circuit and the other of said electrode means; a non-toxic, non-irritant protective covering encasing said electrical conducting means, said capacitance means, and said pulse producing circuit; and said covering including a first member at least partially encompassing said spaced electrode means and a second member for encompassing at least that part of the nerve adapted to be positioned between said electrode means.

11. The apparatus of Claim 10 in which: said electrode means comprise partial cylinders of non-toxic, non-irritant electrically conducting material; and each of said electrode means include a longitudinally extending slot extending from the length of said cylinder and at least initially dimensioned to permit atraumatic passage of the nerve therethrough.

12. The apparatus ofClaim 10 in which said timed electrical pulse producing circuit includes; battery means: a first transistor having first emitter, collector and base electrodes; a second transistor having second emitter, collector and base electrodes; voltage divider means connected across said battery; said first base electrode connected to said voltage divider means; said first emitter electrode connected to one terminal of said battery; said second emitter electrode connected to the other terminal of said battery; impedance means connecting said first collector electrode to said second base electrode; impedance means connecting said first emitter electrode to said second collector electrode; further capacitance means connected between said first base electrode and said second collector electrode; said second collector electrode connected to said capacitance means; and said second emitter electrode connected to said one of said pair of spaced electrode means.

13. The apparatus of Claim 12 in which said pulse producing circuit includes a battery charging circuit connected to said battery means comprising: a secondary electromagnetic induction coil; rectifier means; further electrical conducting means connecting said secondary coil to said rectifier means and said rectifier means to said battery means; and said secondary coil, said rectifier means and said further electrical conducting means adapted to be implanted in the animal and encased in said protective covering.

14. The apparatus of Claim 13 including means external to said animal for providing energy to the battery charging circuit comprising: electromagnetic induction means including a primary electromagnetic induction coil adapted to be placed external to said animal in inductive relation to said implanted secondary electromagnetic induction coil.

15. The method of stimulation ofa selected nerve in a living animal including the steps of: implanting an electrical circuit in the animal; connecting at least a pair of electrodes to the electrical circuit; atraumatically attaching the electrodes to spaced points on the exterior surface of the nerve; energizing the electrical circuit to apply timed stimulating electrical pulses to the nerve of a magnitude sufficient to trigger the nerve for increasing the impulse frequency of the nerve; and sub C. atraumatically coupling the modified pulses to each carotid sinus nerve of the animal.

17. The method of Claim 16 including the step of atraumatically coupling the modified pulses to each carotid sinus nerve of the animal.

18. The method of Claim 16 including the step ofatraumatically coupling the modified pulses to a carotid sinus nerve through bipolar electrodes. 

